The invention relates to a system and method for performing magnetic resonance imaging and, more particularly, to a system and method for acquiring images using magnetic resonance imaging that provide improved, clinically-valuable, magnetic-resonance images of vascular structures.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Magnetic resonance angiography (MRA) and, related imaging techniques, such as perfusion imaging, use the NMR phenomenon to produce images of the human vasculature or physiological performance related to the human vasculature. There are three main categories of techniques for achieving the desired contrast for the purpose of MR angiography. The first general category is typically referred to as contrast enhanced (CE) MRA. The second general category is phase contrast (PC) MRA. The third general category is time-of-flight (TOF) or tagging-based MRA.
Contrast-enhanced MRA techniques require venous cannulation and the use of exogenous contrast material. Such agents are costly and expose the patient to added safety risks, namely, nephrogenic systemic fibrosis. Non-enhanced techniques for MRA are helpful for the evaluation of suspected vascular disease in patients with impaired renal function, since they avoid the risk of nephrogenic systemic fibrosis. Examples of newer non-enhanced techniques include Quiescent-Inflow Single-Shot (QISS) MRA, fresh blood imaging, and flow-sensitive dephasing, such as described in co-pending U.S. application Ser. No. 12/574,856, which is incorporated herein by reference in its entirety.
Many of these techniques use an undersampled Cartesian k-space trajectory combined with parallel imaging to reduce echo train length. However, at 1.5 Tesla Cartesian undersampling factors larger than two to four typically produce poor image quality. Specifically, existing methods for non-enhanced MRA include TOF, fresh blood imaging (FBI), quiescent inflow single shot (QISS), and PC imaging techniques. Unfortunately, TOF produces nondiagnostic image quality outside of the head and neck. FBI and QISS both use Cartesian kspace trajectories and, thus, undersampling factors are limited to about four fold.
In certain circumstances, it would be helpful if higher undersampling factors could be used. For instance, a shortened echo train might be needed for patients with fast heart rates. With sufficiently short echo trains, it might even be possible to acquire data from more than one slice within each heartbeat interval, thereby reducing scan duration. Another potential benefit is that shortening the echo train could reduce sensitivity to respiratory motion or blood flow artifacts. Additionally, one could reconstruct subsets of data that demonstrate different tissue contrast properties (e.g. degree of fat suppression or vascular enhancement) compared with images reconstructed from the entirety of the data.
It is well known that radial k-space trajectories permit the use of high undersampling factors without loss of spatial resolution. However, the data must be sparse in order to minimize radial streak artifacts. To achieve this, highly undersampled radial MRA use image subtraction (e.g. post-contrast-pre-contrast, or flow-rephased-flow-dephased) in order to create “sparse data sets.” However, the process of image subtraction doubles scan time by because one must collect the two data sets necessary to perform the subtraction. Also, by requiring two separate acquisition, the possibility of misregistration artifact is introduced. Furthermore, the most common way to acquire image sets suitable for subtraction is to acquire and subtract a contrast-enhanced and non-contrast-enhanced data set, which reintroduces the drawback of using contrast agents and the implication of nephrogenic systemic fibrosis.
Therefore, it would be desirable to have a system and method for clinical use that is not limited in the way that the above-described and other available techniques are limited.